Procedure and apparatus for producing an a.c. voltage

ABSTRACT

A procedure and an apparatus for producing an a.c. voltage, in which procedure the a.c. voltage (u o ) is produced via inversion from a d.c. voltage (U A ). The d.c. voltage (U A ) is produced by a generator (4). Moreover, the d.c. voltage (U A ) is measured and stabilized. The frequency of the a.c. voltage is determined by means of an oscillator (9).

This application is a continuation of application Ser. No. 368,374 filedJune 23, 1989, is now abandoned.

BACKGROUND OF THE INVENTION

The present invention concerns a procedure and apparatus for producingan a.c. voltage.

In particular in small combustion engine-driven generators producing asinusoidal a.c. voltage, it is difficult to manage simultaneously bothconstant frequency and amplitude under changing load conditions becausethe frequency, and often also amplitude, are determined by speed ofrotation exclusively. The speed is controlled using a centrifugalregulator or another mechanical governor, which in order to reactrequires a fairly large change in speed of rotation. Moreover, exactattainment of the correct frequency often requires separate measurementand continuous monitoring. Achieving a good sinusoidal waveform requiresshaping the generator windings and air gaps in a way deviating from adesign which is required to obtain maximum output power. As a result,the bulk of the generator increases and the weight increases in relationto the power output achieved.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate the drawbacksmentioned above. The procedure of the invention for producing an a.c.voltage is mainly characterized in that the a.c. voltage is formed viainversion from a d.c. voltage produced by means of a generator, that thed.c. voltage is measured and stabilized, and that the frequency of thea.c. voltage is generated with an oscillator. Since the frequency of thea.c. voltage is thus produced with a separate oscillator, a constantfrequency independent of load and the speed of rotation of the generatoris obtained. As the amplitude of the a.c. voltage is at the same timeproduced separately independent of frequency, stabilizing the amplitudebecomes easier.

An advantageous embodiment of the procedure of the invention ischaracterized in that the d.c. voltage is stabilized by controlling themagnetization of the generator.

An advantageous embodiment of a procedure according to the invention isalso characterized in that the oscillator uses a crystal for frequencystabilization.

An advantageous embodiment of the procedure of the invention is alsocharacterized in that the a.c. voltage is filtered in conjunction withthe a.c. conversion.

The apparatus implementing the procedure of the invention ischaracterized in that the apparatus comprises an inverter containing acontrol unit for producing the a.c. voltage via inversion from a d.c.voltage, a generator unit for producing a d.c. voltage by means of agenerator, a d.c. voltage control unit for measuring and stabilizing thed.c. voltage obtained from the generator unit, and an oscillator unit inthe control unit of the inverter for determining the frequency of thea.c. voltage.

An advantageous embodiment of the apparatus implementing the procedureof the invention is characterized in that the magnetization of thegenerator can be controlled by means of the magnetizing winding so as tostabilize the output voltage of the generator unit.

An advantageous embodiment of the apparatus implementing the procedureof the invention is also characterized in that the d.c. voltage controlunit contains an operational amplifier for comparing the d.c. voltageproduced in the generator unit to a reference voltage, transistors whichtogether with the operational amplifier, amplify the difference betweenthe d.c. voltage and the reference voltage, and a diode for excessvoltage protection.

An advantageous embodiment of the apparatus implementing the procedureof the invention is also characterized in that the power stage of theinverter comprises semiconductor switches for producing an a.c. voltagefrom the d.c. voltage and a low-pass filter for filtering the a.c.voltage.

An advantageous embodiment of the apparatus implementing the procedureof the invention is also characterized in that the inverter power stagecomprises an energy storage device connectable to the d.c. voltagecircuit.

An advantageous embodiment of the apparatus implementing the procedureof the invention is also characterized in that the inverter control unitcomprises a crystal and a microcircuit constituting the oscillator, alow-pass filter for producing a sinusoidal signal from the pulse-shapedsignal produced in the oscillator, another low-pass filter for producinga triangular signal from another pulse-shaped signal obtained from theoscillator, a comparator for comparing the sinusoidal and triangularsignals, and a transformer for supplying the signal from the comparatorto the semiconductor switches of the inverter power stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following in detail with the aid of anexample referring to the drawings attached, in which

FIG. 1 shows the apparatus of the invention for producing an a.c.voltage.

FIG. 2 shows the control unit and the magnetizing winding of the d.c.voltage.

FIG. 3 shows the inverter control unit.

FIG. 4a illustrates a square-wave obtained from the oscillator of thecontrol unit.

FIG. 4b illustrates the sinusoidal wave obtained from the low-passfilter of the inverter control unit.

FIG. 4c illustrates the second square wave obtained from the oscillatorof the inverter control unit.

FIG. 4d illustrates the triangular wave obtained from the secondlow-pass filter of the inverter control unit.

FIG. 4e illustrates a rectangular signal modulated with a sinusoidalwave.

FIG. 5 illustrates the power stage of the inverter.

FIG. 1 shows the apparatus which produces the a.c. voltage u_(o) asprovided by the invention by determining the amplitude, frequency andcurve shape of the voltage by means of separate units. The a.c. voltageu_(o) is formed in an inverter 3 by a.c. conversion of the d.c. voltageU_(A). The d.c. voltage U_(A) is produced in a generator unit 1comprising an a.c. generator (G) 4 and a rectifier 5 composed of diodes,which rectifies the a.c. voltage produced by the generator 4. Thegenerator 4 can be e.g. an automobile alternator.

The d.c. voltage U_(A) obtained from the generator unit 1 is measured bythe d.c. voltage control circuit 2 and stabilized by supplying themagnetizing winding L₁ of the generator 1 with a signal produced in thecontrol circuit 2. The frequency of the a.c. voltage u_(o) is formedwith the aid of an oscillator in the control circuit 6 of the inverter.

FIG. 2 is a more detailed view of the d.c. voltage control unit 2 andthe magnetizing winding L₁ presented in FIG. 1. The d.c. voltage U_(A)produced by the generator unit 1 is applied to a voltage dividercomposed of resistors R₁ and R₂ in which the voltage U_(A) is adapted toa level appropriate for input to the operational amplifier 8. The otherinput of the operational amplifier 8 is fed by a reference voltageformed from the voltage U_(B) by means of the resistor R₃ and the Zenerdiode Z₁. The difference between the two input voltages of theoperational amplifier 8 is amplified by the operational amplifier 8 andtransistors T₁ and T₂.

The base electrode of transistor T₁ is controlled by connecting thevoltage U_(B) to resistor R₄, the base electrode of transistor T₁connected over transistor T₂ and resistor R₅. The other terminal ofresistor R₄ is connected to the base electrode of transistor T₁, andtransistor T₁ is controlled by connecting the output of the operationalamplifier 8 to its base electrode. The collector of transistor T₁supplies the magnetizing winding L₁. Parallel to the magnetizing windingL₁ is connected a diode D₁, which inhibits access of overvoltage peaksto transistor T₂. The voltage U_(B) has a magnitude of 12 V, obtainedfrom a storage battery or the generator unit 1.

FIG. 3 shows the control unit 6 of the inverter depicted in FIG. 1 ingreater detail. The control unit is provided with an oscillator 9 fordetermining the frequency of the output a.c. voltage u_(o). Theoscillator consists of a crystal 10 with which the frequency isstabilized, and of a microcircuit 11 containing a phase inverter. IfCMOS circuit CD4060 is used as the microcircuit 11, a suitable frequencyof the crystal 10 is 819.2 kHz. When the signal obtained from thecrystal 10 is divided in the microcircuit 11, a square wave signal S₁,shown in FIG. 4a, is obtained which has the frequency (50 Hz) requiredfor the inverter output voltage u_(o). By dividing the signal obtainedfrom the crystal 10 by 32, another square wave S₂ is obtained, depictedin FIG. 4b, which has the frequency of 25.6 kHz. The signal S₁ isobtained by further dividing the second signal S₂ more by 512. Thesignal S₁ is fed into the low-pass filter 12 which passes thefundamental frequency 50 Hz but does not pass any higher harmonics,whereby a sinusoidal signal S₃ as shown in FIG. 4c is produced.

The low-pass filter 12 can be implemented using an operational amplifiercircuit, the design of which is obvious to a person skilled in the artand is therefore not dealt with in the present context. The other squarewave S₂ is fed to another low-pass filter 13 composed of a resistor R₆and a capacitor C₁, from the output of which is obtained the triangularwave S₄ shown in FIG. 4d. The sinusoidal signal S₃ and the triangularsignal S₄ are compared in the comparator 14, which outputs the signal S₅shown in FIG. 4e. This is a 25.6 kHz square signal modulated with a 50Hz sinusoidal wave. This signal is applied via an amplifier 15 to atransformer M₁ which supplies the power stage 7 of the inverter. Thecapacitor C₂ separates the 50 Hz component present in the signal. Thewaves in FIGS. 4a-4e do not coincide in scale or in phase: they merelyrepresent the signal form in each case.

FIG. 5 shows the power stage 7 of the inverter depicted in FIG. 1, inwhich the voltage U_(A) is the stabilized d.c. voltage formed in thegenerator unit 1, depicted in FIG. 1. The windings M₂ -M₅ are secondarywindings of the transformer M₁ presented in FIG. 3, while the capacitorsC₃ -C₆ are decoupling capacitors, each of which, together with therespective VDR resistors R₇ -R₁₀, reconstructs the modulated signal S₅formed in the comparator 14. Instead of VDR resistors, Zener diodes canalso be used. The windings M₂ -M₅ are so connected that windings M₂ andM₅ have the same polarity while windings M₃ and M₄ are reversed inpolarity relative to that of M₂ and M₅. A reconstructed signal is usedto drive each of the MOSFET semiconductor switches K₁ -K₄.

At points A and B, a signal is produced which corresponds in form to thesignal S₅ in FIG. 4e and in amplitude to the voltage U_(A). The signalsat the points A and B are opposite in phase. By passing said signalsthrough a second order low-pass filter, consisting of an inductance L₂,a capacitor C₇ and another inductance L₃ connected in series, an outputvoltage u_(o) is obtained across the terminals of the capacitor C₇. Thevoltage u_(o) is sinusoidal, its frequency is stabilized with thecrystal 6, and its amplitude corresponds to the d.c. voltage U_(A). Thecapacitor C₈ is an electrolytic capacitor which enables a highermomentary output power to be achieved than could be obtained from thegenerator alone.

It is obvious to a person skilled in the art that the differentembodiments of the invention are not restricted to the example describedin the foregoing, and that they may vary within the scope of the claimsstated below. A three-phase a.c. voltage may be formed by connectingthree inverters in parallel. Instead of MOSFET semiconductor switches,other kinds of corresponding semiconductor switches may be used whichhave sufficient switching speed for a 25.6 kHz signal, and which areable to sustain the voltage U_(A) and current drawn by the loadconnected to the output of the inverter 3. Besides a combustion engine,e.g. a hydraulic turbine, wave power or a windmill may be used to drivethe generator 4 in FIG. 1. The capacitor C₈ of FIG. 5 may also be astorage battery or another equivalent energy storage, in which case atransient stopping of the generator 4 does not interrupt the operationof the entire apparatus. The 50 Hz frequency of the output voltage u_(o)can also be changed e.g. into 60 Hz by replacing the crystal 10 in FIG.3 with a 983.04 kHz crystal.

We claim:
 1. Apparatus for producing an a.c. voltage, comprising:an a.c.generator for producing a first a.c. voltage, rectifier means forrectifying said a.c. voltage to produce as an output a d.c. voltage,inverter means for inverting said d.c. voltage to produce as an output asecond a.c. voltage, said second a.c. voltage having a substantiallyconstant frequency and a substantially constant maximum amplitude andbeing independent from a rotational velocity of said a.c. generator,control means for stabilizing said d.c. voltage to be substantiallyconstant by measuring said d.c. voltage and regulating magnetization ofsaid a.c. generator so that the amplitude of said second a.c. voltage isregulated, wherein said inverter means includes oscillator means forstabilizing the frequency of said second a.c. voltage so that it isformed separately from the amplitude of said second a.c. voltage. 2.Apparatus according to claim 1, wherein said inverter means includes aplurality of semiconductor switches which are fully gate controlled, andsaid oscillator means being connected for controlling said semiconductorswitches, said means for controlling said semiconductor switchesincluding said oscillator means, said oscillator means producing a firstpulse-shaped signal and a second pulse-shaped signal which is differentfrom said first pulse-shaped signal, said oscillator means including acrystal and a microcircuit, said inverter means further comprising (a) alow-pass filter for producing a sinusoidal signal from said firstpulse-shaped signal generated by said oscillator means, (b) a secondlow-pass filter for forming a triangular signal from said secondpulse-shaped signal generated by said oscillator means, (c) comparatormeans for comparing said sinusoidal signal and said triangular signaland for producing a comparator output signal, and a transformer meansfor supplying said comparator output signal to each of said plurality ofsemiconductor switches.
 3. Apparatus according to claim 1, furthercomprising filter means connected to said plurality of semiconductorswitches for filtering an output of said plurality of semiconductorswitches to produce said second a.c. voltage.
 4. A method of producingan a.c. voltage from a first a.c. voltage produced by an a.c. generator,comprising the steps of:rectifying said first a.c. voltage with arectifier to produce a d.c. voltage; measuring said d.c. voltage andstabilizing said d.c. voltage by controlling magnetization of said a.c.generator based on the measured said d.c. voltage; inverting said d.c.voltage to produce a second a.c. voltage with an inverter means having aplurality of semiconductor switches such that said second a.c. voltagehas a substantially constant frequency and amplitude which areindependent from a rotational velocity of said a.c. generator, said stepof inverting comprising the steps of (a) providing an oscillator meansproducing an oscillator output signal and (b) using said oscillatoroutput signal to convert said d.c. voltage into said second a.c. voltagesuch that said second a.c. voltage has a separately formed amplitude andfrequency which are substantially constant.
 5. A method according toclaim 4, wherein said step of controlling magnetization of said a.c.generator comprises the steps of: (a) obtaining a reference voltage fromone of an accumulator or said a.c. generator, (b) comparing said d.c.voltage to said reference voltage to form a difference signal, and (c)supplying said difference signal to a magnetizing winding of said a.c.generator.
 6. A method according to claim 4, wherein in said step ofinverting said d.c. voltage, further comprising the steps of: providingsaid semiconductor switches to be fully gate controllable, providingsaid oscillator to have a crystal and a microcircuit, using saidoscillator to produce a first pulse-shaped signal and a secondpulse-shaped signal, using said oscillator for controlling saidplurality of semiconductor switches, and providing said inverter meansto have a low-pass filter, using said low pass filter to produce asinusoidal signal from said first pulse-shaped signal generated by saidoscillator, a second low-pass filter for forming a triangular signalfrom said second pulse-shaped signal generated by said oscillator, acomparator for comparing said sinusoidal and triangular signals toproduce a comparator output signal, and a transformer for supplying saidcomparator output signal to said semiconductor switches.
 7. Electricalpower system for producing sinusoidal a.c. voltage consisting of an a.c.generator provided with a magnetization winding for producing a firsta.c. voltage, rectifier for rectifying said first a.c. voltage to a d.c.voltage, gate controlled semiconductor switches for inverting said d.c.voltage to a second a.c. output voltage, a first control unit forcontrolling said switches, said control unit comprising an oscillatorprovided with stabilizing means for determining the frequency of saidoutput voltage independently from the rotational velocity of saidgenerator, a second control unit for stabilizing the amplitude of saidoutput a.c. voltage by measuring the d.c. voltage and regulating themagnetization of said generator in order to get an essentially constantd.c. voltage determining said amplitude, and filter means connected tosaid switches for forming an essentially sinusoidal output voltage. 8.Electrical power system according to claim 7, wherein said first controlunit consists of a crystal or alike and a microcircuit, constituting anoscillator, a low-pass filter for producing a sinusoidal signal from apulse-shaped signal generated by the oscillator, a second low-passfilter for forming a triangular signal from another pulse-shaped signalobtained from the oscillator, a comparator for comparing the sinusoidaland triangular signals, and a transformer for supplying the signalobtained from the comparator to the semiconductor switches. 9.Electrical power system according to claim 7, wherein the second controlunit regulates the magnetization of said a.c. generator by comparingsaid d.c. voltage to a reference voltage, which is formed from anaccumulator or said a.c. generator, and leading the difference of thecomparison to the magnetizing winding of said a.c. generator. 10.Electrical power system according to claim 7, wherein said semiconductorswitches are fully gate controlled, arranged in a H-bridge configurationand controlled with pulse width modulation.